Deck 13: Structure Determination

Question 1

Instructions: Match each of the following groups of bond-types to the region of the infrared spectrum in which their absorptions occur.

Answer 1

Question 2

Below is the mass spectrum of an unknown hydrocarbon. In addition, this hydrocarbon shows characteristic absorption at 2100 cm^-1 in its IR spectrum. Give the structure of this unknown. (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)
The formula weight of 54 corresponds to a molecular formula of C₄H₆, which has two degrees of unsaturation. Possible structures for this formula are:

Answer 2

The IR data is consistent with an internal alkyne, so the compound is but-2-yne.

Question 3

Which of the following bonds undergoes stretching at the highest frequency?

A) C=O
B) C-O
C) C=C
D) C-C

Answer 3

A

Question 4

The amount of energy in electromagnetic radiation is related to the frequency and wavelength of the radiation. High energy radiation, like gamma rays, is of:

A) low frequency and short wavelength
B) low frequency and long wavelength
C) high frequency and short wavelength
D) high frequency and long wavelength

Question 5

Examining the infrared spectrum of a compound allows us to:

A) determine the types of functional groups present in the compound
B) determine the carbon-hydrogen framework of the compound
C) determine the molecular weight of the compound
D) determine the nature of the conjugated pi electron system in the compound

Question 6

What are the units for electromagnetic radiation used in infrared spectroscopy?

A) cm
B) cm^-1
C) J.mol^-1
D) none, wavenumber is a dimensionless quantity

Question 7

Cyclohexene and hex-2-yne both have the molecular formula, C₆H₁₀.
a) How would you use infrared spectroscopy to distinguish between the two compounds
b) How could the mass spectrum be used to distinguish between the two compounds?

Question 8

At what approximate positions might the compound below show IR absorptions?

Question 9

Which of the following bonds undergoes stretching at the highest frequency?

A) O-H
B) C-H
C) C=O
D) CºC

Question 10

Instructions: Refer to the mass spectrum of 2-methylbutane shown below to answer the following question(s). (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)
Refer to instructions. What peak represents the base peak?

Question 11

Assume you are carrying out the conversion of 1-bromobutane to butan-1-ol. How could you use IR spectroscopy to determine when the reaction is complete?

Question 12

Instructions: Select the most reasonable formula for the compounds with the following mass spectral data. Refer to instructions. M⁺ at m/z = 216

A) C₆H₁₃OCl
B) C₄H₈Br₂
C) C₁₀H₁₆
D) C₉H₁₂O

Question 13

Which of the following regions in the electromagnetic spectrum corresponds to the radiation with the highest energy?

A) radio waves
B) ultraviolet
C) infrared
D) visible

Question 14

The amount of energy in infrared light corresponds to:

A) the amount of energy needed to promote one electron from a bonding to an antibonding molecular orbital
B) the amount of energy needed to fragment a molecule
C) the amount of energy needed to strip a molecule of one electron to generate a cation radical
D) the amount of energy needed to increase certain molecular motions, such as bond vibrations, in organic molecules

Question 15

Which of the following statements best describes the base peak in a mass spectrum?

A) The peak from the most stable radical.
B) The peak from the species that has the isotope with the highest atomic number.
C) The peak of highest intensity.
D) The peak from the molecule minus an electron.
E) The M +1 peak

Question 16

Instructions: Match a structure from the list below to the following IR spectra.
A.


B.


C.


D.


E.


F.

Question 17

Which of the following compounds gives an infrared spectrum with peaks at 3300 cm^-1 (strong, broad peak) and 1640 cm^-1 (sharp, weak peak)?

A)
B)
C)
D)

Question 18

Consider the compound below:

Question 19

Instructions: Refer to the mass spectrum of 2-methylbutane shown below to answer the following question(s). (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)
Refer to instructions. What peak represents M⁺?

Question 20

Which of the following does not involve the interaction of molecules with electromagnetic energy?

A) mass spectrometry
B) infrared spectroscopy
C) ultraviolet spectroscopy
D) nuclear magnetic resonance spectroscopy

Question 21

When 2-bromopropane reacts with ethoxide ion, two products are formed; one is the product of S_N2 substitution and the other is the product of E2 elimination. Write the structures of both products, and tell how they could be distinguished using IR spectroscopy.

Question 22

Which of the following compounds would show the longest wavelength l_max in its UV spectrum?

Question 23

When an organic molecule is irradiated with ultraviolet radiation, the energy absorbed by the molecule corresponds to:

Question 24

What is the horizontal axis of a mass spectrum?

A) mass
B) mass/energy
C) mass/charge
D) charge

Question 25

Instructions: The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description.
Refer to instructions. __________ When looking at an NMR chart the right-hand part of the chart is the __________.

Question 26

Circle any of the following compounds that would be a candidate to produce a UV absorption spectrum.

Question 27

What is the vertical axis of a mass spectrum?

A) mass
B) energy
C) abundance
D) field strength

Question 28

Which of the following compounds gives an infrared spectrum with peaks at 3000-3500 cm^-1 and ~1750 cm^-1?

A)
B)
C)
D)

Question 29

Instructions: The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description.
Refer to instructions. __________ The calibration standard for ¹H and ¹³C NMR is:

Question 30

A 3.42 ´ 10^-5 M solution of dibenzalacetone in ethanol produced an absorbance of 0.753 in a 1.00 cm cell. Based on this data, what is e for this compound?

A) 2.57 ´ 10⁵
B) 2.20 ´ 10⁴
C) 4.54 ´ 10^-5
D) 2.92 ´ 10⁴

Question 31

Instructions: The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description.
Refer to instructions. __________ The NMR charts are calibrated using an arbitrary scale that is divided into __________ units.

Question 32

Which of the following bonds gives rise to a strong absorbance near 1700 cm^-1 in the infrared spectrum?

A) C=O
B) C-O
C) C=C
D) C-C

Question 33

Instructions: The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description.
Refer to instructions. __________ The exact place on the chart at which a nucleus absorbs is called its __________.

Question 34

Instructions: The following questions pertain to the charting of NMR spectra. MATCH a term to each description below. Place the letter of the term in the blank to the left of the description.
Refer to instructions. __________ The NMR charts are calibrated using an arbitrary scale that is divided into __________ units.

Question 35

Which of the following compounds gives an infrared spectrum with peaks at 3300 cm^-1 (sharp peak) and 2150 cm^-1 (sharp peak)?

A) $$\mathrm { CH } _ { 3 } \mathrm { CH } _ { 2 } \mathrm { C } \equiv \mathrm { CH }$$
B) $$\mathrm { CH } _ { 3 } \mathrm { C } \equiv \mathrm { CCH } _ { 3 }$$
C)
D)

Question 36

Which of the following compounds gives an infrared spectrum with a peak at ~1750 cm^-1, but no significant peaks at 3000-3500 cm^-1 or 1050-1250 cm^-1?

A)
B)
C)
D)

Question 37

Nuclear magnetic resonance spectroscopy provides information about a molecule's:

Question 38

Which type of spectroscopy (IR, UV, or MS) will best distinguish between the pair of compounds below? Give a brief reason.

Question 39

Which of the following compounds would have the longest l_max in the UV region of the electromagnetic spectrum?

A) A
B) B
C) C
D) All would have the same l_max.

Question 40

Which of the following compounds gives an infrared spectrum with a peak at strong 1730 cm^-1?

A)
B)
C)
D)

Question 41

Treatment of tert-butyl alcohol with hydrogen chloride yields a mixture of tert-butyl chloride and 2-methylpropene.

Question 42

Instructions: Predict the splitting patterns you would expect for each proton in the molecules below:
Predict:

Question 43

Instructions: Predict the splitting patterns you would expect for each proton in the molecules below:
Predict:

Question 44

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.
Number of signals:

Question 45

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.
Number of signals:

Question 46

Which feature in the ¹H NMR spectrum provides information about the number of neighboring protons of each proton in the compound?

A) number of signals
B) integration of signals
C) splitting of signals
D) chemical shift

Question 47

A compound with the molecular formula C₅H₁₂O produces only two singlets in the ¹H NMR spectrum.

Question 48

Instructions: Refer to the structure of 3-methylbutan-2-one below to answer the following question(s). Refer to instructions. What is the ratio of peak areas upon integration of the spectrum for A, B, and C respectively?

A) 3:1:3:3
B) 1:1:6
C) 1:1:6
D) 3:1:6

Question 49

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.
Number of signals:

Question 50

Which feature in the ¹H NMR spectrum provides information about the electronic environment of the protons in a compound?

A) number of signals
B) integration of signals
C) splitting of signals
D) chemical shift

Question 51

Which of the following compounds would produce the most downfield signal in a ¹³C NMR spectrum?

A) A
B) B
C) C
D) D

Question 52

Which of the following would produce only singlets in an ¹H NMR spectrum?

A) A
B) B
C) C
D) D
E) all of these produce only singlets

Question 53

Which feature in the ¹H NMR spectrum provides information about the relative numbers of hydrogen atoms of each type found in a compound?

A) number of signals
B) integration of signals
C) splitting of signals
D) chemical shift

Question 54

Instructions: Answer the following question(s) for the compound whose ¹H NMR spectra is shown below.
C₄H₈O (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)
Refer to instructions. Propose a structure for this compound.

Question 55

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.
Number of signals:

Question 56

Instructions: Answer the following question(s) for the compound whose ¹H NMR spectra is shown below.
C₄H₈O (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)
Refer to instructions. Describe each signal in terms of its integration, splitting and chemical shift.

Question 57

A compound with the molecular formula C₆H₄ClBr produces only two doublets in the ¹H NMR spectrum.

Question 58

Which structure of molecular formula C₄H₈Cl₂ fits the ¹H NMR and ¹³C NMR spectra shown below? (Spectrum obtained from: SDBSWeb: http://www.aist.go.jp/RIODB/SDBS/)

A)
B)
C)
D)

Question 59

Instructions: Refer to the structure of 3-methylbutan-2-one below to answer the following question(s). Refer to instructions. What is the splitting pattern for the hydrogens in 3-methylbutan-2-one labeled A, B, and C, respectively?

A) singlet, singlet, singlet
B) singlet, septet, quartet
C) singlet, septet, doublet
D) singlet, septet, doublet, doublet

Question 60

Instructions: For each of the compounds below tell how many signals you would expect the molecule to have in its normal, broadband decoupled ¹³C NMR spectra.
Number of signals:

Question 61

Which of the following combinations of peaks appears in the ¹H NMR spectrum of diethyl ether, CH₃CH₂OCH₂CH₃?

A) a triplet and a doublet
B) a quartet and a sextet
C) two singlets
D) a triplet and a quartet

Question 62

Which of the following combinations of peaks appears in the ¹H NMR spectrum of 2-methylpropane?

A) two singlets
B) a singlet and a nonet
C) a singlet and a decet
D) a doublet and a decet